A series of HTLV-I cell lines were prepared by in vitro transformation of rabbit peripheral blood mononuclear cells (PBMC) using HTLV-I cell lines of human or rabbit origin as virus source. It was found that certain infected lines killed rabbits in a dose dependent manner; animals injected with the lethal lines developed high temperature within four days of injection and usually died by day 8. A detailed description of organs from the affected animals is in progress; it appears that the HTLV-I lines cause infiltration of lung, lymph nodes, spleen and liver in a manner similar to adult T cell leukemia lymphoma in humans. Injection of irradiated cells was not lethal within the same short time frame as live cells, but most hallmarks of the disease were present. The majority of rabbit HTLV-I cell lines including several derived in the same fashion as the lethal line cause no overt disease. A detailed study of this phenomenon is underway; results here can shed light on why HTLV-I infection in humans causes serious disease in only a small percentage of infected individuals whereas most infected persons remain asymptomatic. Progress has been made in comparing lethal and nonlethal cell lines including a) structure of virus in lethal and non lethal cell lines; b) Characterization of the lines as CD4-, CD8-, gammadelta TCR T cells c) disease caused by lethal lines has been shown to be similar to human adult T cell leukemia/ lymphoma; d) host responses to lethal vs. nonlethal lines show difference in immune reactivity at the cellular level.